Magnetic memories, particularly magnetic random access memories (MRAMs), have drawn increasing interest due to their potential for high read/write speed, excellent endurance, non-volatility and low power consumption during operation. An MRAM can store information utilizing magnetic materials as an information recording medium. One type of MRAM is a spin transfer torque random access memory (STT-MRAM). STT-MRAM utilizes magnetic junctions written at least in part by a current driven through the magnetic junction. A spin polarized current driven through the magnetic junction exerts a spin torque on the magnetic moments in the magnetic junction. As a result, layer(s) having magnetic moments that are responsive to the spin torque may be switched to a desired state.
For example, FIG. 1 depicts a conventional magnetic tunneling junction (MTJ) 10 as it may be used in a conventional STT-MRAM. The conventional MTJ 10 typically resides on a bottom contact 11, uses conventional seed layer(s) 12 and includes a conventional antiferromagnetic (AFM) layer 14, a conventional reference layer 16, a conventional tunneling barrier layer 18, a conventional free layer 20, and a conventional capping layer 22. Also shown is top contact 24.
The conventional seed layer(s) 12 are typically utilized to aid in the growth of subsequent layers, such as the AFM layer 14, having a desired crystal structure. The conventional tunneling barrier layer 18 is nonmagnetic and is, for example, a thin insulator. For a higher signal, the conventional tunneling barrier 18 is typically crystalline MgO.
The conventional reference layer 16 and the conventional free layer 20 are magnetic. The magnetization 17 of the conventional reference layer 16 is fixed, or pinned, in a particular direction, typically by an exchange-bias interaction with the magnetization of AFM layer 14. Further, other versions of the conventional MTJ 10 might include an additional reference layer (not shown) separated from the free layer 20 by an additional nonmagnetic barrier or conductive layer (not shown). The conventional free layer 20 has a changeable magnetic moment 21. Although depicted as a simple layer, the conventional free layer 20 may also include multiple layers. Typically, materials such as CoFeB are used in the conventional free layer 16 and/or reference layer 20.
To switch the magnetic moment 21 of the conventional free layer 20, a current is driven perpendicular to plane (in the z-direction). When a sufficient current is driven from the top contact 24 to the bottom contact 11, the magnetic moment 21 of the conventional free layer 20 may switch to be parallel to the magnetic moment 17 of the conventional reference layer 16. When a sufficient current is driven from the bottom contact 11 to the top contact 24, the magnetic moment 21 of the free layer may switch to be antiparallel to that of the reference layer 16. The differences in magnetic configurations correspond to different magnetoresistances and thus different logical states (e.g. a logical “0” and a logical “1”) of the conventional MTJ 10. Thus, by reading the tunneling magnetoresistance (TMR) of the conventional MTJ 10 the state of the conventional MTJ can be determined.
FIG. 2 is a high level flow chart depicting a conventional method 50 for fabricating the conventional magnetic junction 10. The layers 12, 14, 16, 18, 20 and 22 for the magnetic junction 10 are deposited, via step 52. Typically a full film deposition is used. The conventional magnetic junction 10 is defined, via step 54. For example, the conventional magnetic junction 10 may be covered by a mask and exposed portions of the layers 12, 14, 16, 18, 20 and 22 removed via ion milling.
As part of fabrication, the conventional magnetic junction 10 is also annealed, via step 56. The annealing may be used to ensure that the layers of the conventional MTJ 10 have the desired crystal structure. For example, the MgO in the conventional tunneling barrier layer 18 is typically amorphous as deposited. In addition, the reference layer 16 and/or free layer 20 typically include a CoFeB layer, with up to twenty percent of B. These layers 16 and/or 20 may also be amorphous as-deposited. In order for the layers 16, 18 and 20 in the conventional MTJ 10 to have the desired crystal structure and crystallographic orientation, the conventional MTJ 10 is annealed in step 56. The annealing of the conventional MTJ 10 is typically carried out at a temperature of approximately three hundred degrees Celsius.
Although the conventional MTJ 10 may function, there are drawbacks to incorporating the conventional MTJ 10 in a memory. For example, even with the annealing carried out in step 56, the conventional MgO tunneling barrier layer 18 may not have the desired crystal structure. For example, the conventional MgO tunneling barrier 18 may not have the desired texture. Similarly, the reference layer 16 and free layer 20 may not be fully crystallized as desired. As a result, the TMR for the conventional MTJ 10 may be reduced.
In addition, as discussed above, the free layer 20 and/or reference layer 16 may include CoFeB. The presence of B in the free layer 20 and/or reference layer 16 aids in crystallization of the conventional tunneling barrier 20. It is believed that the mobility of B in the CoFeB allows the freedom in the lattice of the layers 16, 18 and/or 20 for rearrangement of the amorphous layers into ordered crystal structures. However, the mobility of the B also allows the B to interdiffuse into the MgO tunneling barrier 20. The B may also diffuse to other layers in the stack of the conventional MTJ 10. The presence B in the conventional tunneling barrier 18 and other layers may be detrimental to properties of the conventional MTJ, such as the TMR. Thus, the signal from the conventional MTJ is, again, reduced.
Other characteristics of the conventional MTJ 10 may also be desired to be improved. For example, the critical current is the write current required to be driven through the conventional MTJ 10 to switch the magnetic state of the free layer 16. The critical current of the conventional MTJ 10 may be too high for use in a spin transfer torque magnetic random access memory (STT-MRAM). For example, for use in an STT-MRAM, the critical switching current density may be desired to be less than 1 MA/cm2. Further, although the magnetic moments 17 and 21 of the conventional reference layer 16 and conventional free layer 20 are shown as in plane, these moments may be desired to be perpendicular to plane. Thus, the free layer 20 and/or reference layer 16 may be desired to have a high perpendicular magnetic anisotropy (PMA). A high PMA occurs when the perpendicular anisotropy energy exceeds the out-of-plane demagnetization energy. This results in a magnetic moment that has a component perpendicular to plane and may be fully perpendicular to plane. Although such conventional high PMA junctions do exist, the PMA may be reduced by various factors. For example, PMA may be reduced by Co inclusions into Fe in a CoFe free layer 20, by the presence of B in the conventional free layer 20, as well as other factors. Further, the thermal stability of the conventional free layer 20 may be difficult to maintain using conventional high PMA materials. As a result, performance of the conventional MTJ may suffer. Consequently, mechanisms for tailoring the PMA may also be desired.
Accordingly, what is needed is a method and system that may improve the performance of the spin transfer torque based memories. The method and system described herein address such a need.